The objective of this study was to examine the effect of dietary protein intake on serum levels of triiodothyronine (T3), thyroxine (T4), prolactin, corticosterone, aldosterone, creatinine, and cholesterol in female Fischer 344 rats. Weanling rats were fed diets containing 4, 8, 12 or 24% protein for 14 months prior to the determination of serum hormones, creatinine, and cholesterol levels. Levels of all the variables examined were similar in rats fed the 8, 12, and 24 percent protein diets. Rats fed the 4 percent protein diets had markedly higher T4 and T3 levels, lower corticosterone and aldosterone levels and similar creatinine and cholesterol levels compared to the other three groups.